A method of obtaining the oblique incident sound absorption coefficient through an on-the-spot measurement

A new method (tentatively called the ‘cancellation method’) of determining the sound absorption coefficient is presented. This method makes it possible to obtain the absorption coefficient of a wall material by means of an on-the-spot field measurement which has, until now, been considered very difficult. This paper shows that the reflection from a sample can be obtained by combining the outputs from two non-directional microphones through a phase inverter, and how, by comparing it with the direct sound, measured separately, the sound absorption coefficient can be estimated. Absorption coefficients of several kinds of samples obtained by this method are compared with those calculated theoretically for the locally reactive material.